1. Field of the Invention
The present invention relates to a method of cleaning a copper-INVAR-copper laminate in an acid solution without inducing a galvanic etching of the INVAR.
2. Description of the Related Art
A copper-INVAR-copper laminate is commonly included in electronic packages. For example, a copper-INVAR-copper laminate may be present within a dielectric substrate. Exterior surfaces of the substrate may be circuitized such that a plated through hole (PTH) passes through the copper-INVAR-copper laminate and electrically couples the surface circuitizations. Unfortunately, various acid cleaning steps in the formation of the preceding electrical structure may cause undesirable pocket voids to form in the INVAR layer of the copper-INVAR-copper laminate.
After a through hole is formed in the substrate, such as by laser drilling, and prior to a plating of the through hole to complete formation of the PTH, the exposed copper and INVAR surfaces may be pre-cleaned by immersing the substrate in an acid solution. Such acid pre-cleaning removes oxides previously formed on the copper surfaces and cleans the INVAR surface. A suitable acid for this purpose is, inter alia, a mixture of sulfuric acid and phosphoric acid. The copper-INVAR-copper laminate in an acid solution results in formation of a galvanic cell that etches the INVAR but does not etch the copper, thus forming pocket voids within the INVAR. The electrochemistry that selectively forms the pocket voids will be described infra. The pocket voids, if deep enough, may prevent the INVAR from being copper plated in subsequent copper-plating steps. Even if the pocket voids do not impair subsequent copper plating, the structural integrity of the copper plating within the pocket voids will be weak and subject to rupture from thermal or mechanical stresses.
After the acid pre-cleaning, a substrate surface and the through hole are both plated with copper. The copper plating may be accomplished by first forming a thin porous copper layer by electroless plating, or by coating with another type of conductive material such as a conductive graphite or a conductive polymer, followed by forming a relatively thicker layer of copper by electroplating. The electroless plating of copper may be accomplished by any known process such as by first depositing on the surfaces to be plated an adsorbing material, such as a cationic surfactant, which serves as an attractant for the next material to be deposited, namely a seed material such as palladium. Following the palladium seeding, the substrate is electroless plated with copper.
After the electroless plating of copper and before the electroplating of copper, a cleaning step removes oxides from exposed copper surfaces by immersing the substrate in an acid solution such as a sulfuric acid solution. Inasmuch as the thin copper plating in a through hole is porous, acid may migrate through the pores of the thin copper plating and become trapped between the thin copper plating and the INVAR surface covered by the thin copper plating. Subsequent electroplating of copper seals the trapped acid. As with the acid pre-cleaning described supra, the INVAR in contact with the sealed acid is subject to pocket void formation due to galvanic action. The pocket voids resulting from the cleaning step are typically larger and deeper than are the pocket voids resulting from the pre-cleaning step, since the sealed acid will continuously contact the INVAR for an indefinite period of time. Because of the pocket voids, the structural integrity of the copper plating that covers the pocket voids will be weak and subject to rupture from thermal or mechanical stresses.
FIGS. 1-3 illustrates changes in the INVAR layer of a copper-INVAR-copper laminate within a dielectric substrate after the substrate has been immersed in an acid solution. FIG. 1 shows a front cross-sectional view of a substrate 10 having dielectric material 12, wherein the substrate 10 includes an internal copper-INVAR-copper laminate 20. The copper-INVAR-copper laminate 20 includes an INVAR layer 24 sandwiched between a first copper layer 22 and a second copper layer 26. A prior-drilled through hole 28 passes through the copper-INVAR-copper laminate 20 and exposes surface 32 of the first copper layer 22, surface 34 of the first INVAR layer 24, and surface 36 of the second copper layer 26. FIG. 2 shows the substrate 10 immersed in an acid solution 15 for a purpose such as cleaning the surfaces 32, 34, and 36. FIG. 3 illustrates the substrate 10 after the substrate 10 is removed from the acid solution 15. FIG. 3 shows a pocket void 25 in the INVAR layer that resulted from electrochemical etching of the INVAR layer 24 by galvanic action.
The electrochemistry associated with the etching of the INVAR layer 24 is straightforward. While the substrate 10 is immersed in acid solution 15, as shown in FIG. 2, a first galvanic cell is formed from the first copper layer 22, the INVAR layer 24, and the acid solution 15. In the first galvanic cell, hydrogen ions (H.sup.30) combine reductively with electrons to form hydrogen gas (H.sub.2); i.e. EQU 2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 (1)
wherein the hydrogen ions for Equation (1) are supplied by the acid solution 15, such as by sulfuric acid (H.sub.2 SO.sub.4):
H.sub.2 SO.sub.4.fwdarw.2H.sup.+ +SO.sub.4.sup.-2
The electrons (e.sup.-) for Equation (1) are generated in the INVAR layer 24 and are transported into the first copper layer 22 due to a difference in potential between the first copper layer 22 and the INVAR layer 24. Finally, the electrons flow along the surface 32 of the first copper layer 22 and into the acid solution 15 where the electrons combine with hydrogen ions to form hydrogen gas. Note that there is no net effect on the first copper layer 22, which explains why the first copper layer 22 is not etched.
Noting that INVAR is a trademark for a ferronickel alloy that includes iron (Fe) and nickel (Ni) with a composition of 63.8% iron, 36% nickel, and 0.2% carbon, the two electrons supplied by the INVAR layer 24 result from an oxidation process that forms ionic species from the iron (e.g, Fe.sup.++, Fe.sup.+++) and nickel (e.g, Ni.sup.++). Applicants have not yet determined the ionic species of iron and nickel, and their relative concentrations, that participate in the ionic chemistry of the INVAR layer 24. Nonetheless, the ionic species actually formed from the iron and nickel dissolve in the acid solution 15 and are thus permanently removed from the INVAR layer 24. The vacating ionic species from the INVAR layer 24 leave an empty space in the INVAR layer 24 so as to form the pocket void 25 shown in FIG. 3. Analogous electrochemical transport processes from a second galvanic cell likewise contribute to the formation of the pocket void 25, wherein the second galvanic cell includes the second copper layer 26, the INVAR layer 24, and the acid solution 15.
As stated supra, the structural integrity of the copper plating that covers a pocket void, and is within a pocket void, is weak and subject to rupture from thermal stresses. Additionally, continuous formation of hydrogen gas from galvanic action of sealed acid within a pocket void develops a gas pressure within the pocket void that may either weaken or rupture the copper plating that covers the pocket void. Thus, a method is needed to prevent pocket void formation in an INVAR layer of a copper-INVAR-copper laminate immersed in an acid solution.